CATEGORY A: Hey, I like smoochin' a distant relative too!
CATEGORY B: I think T-Rex is wrong because he forgot to consider siblings, and with siblings two kids can have the same parents!

The bad news for these category B people is that siblings don't actually affect the argument at all. We're tracing the ancestors of one guy (let's call him "T-Rex") and it's really irrelevant whether or not he has siblings: all we care about is the fact that this T-Rex guy necessarily had two genetic parents, and that these parents each had two genetic parents themselves, etc. So when we arrive at ten quadrillion ancestors after fifteen hundred years, that's all just to produce one dude!

I did it this way because the math is way easier. If you wanted to trace it back for a SECOND person (let's call this fellow "Utahraptor"), then yeah, there's a possibility that two sets of parents could overlap. But even that's not any help: if we imagine that T-Rex and Utahraptor are brothers and thus have perfect overlap in their family tree, you still need the ten quadrillion people to produce either of them. And if we take the other extreme and imagine that T-Rex and Utahraptor are completely unrelated then you need ten quadrillion people EACH to get just these two dudes! Twenty quadrillion people! Sheesh.

Anyway the bottom line is that math is not going to help you here; somewhere in your family tree relatives are bangin' away